1. Field of the Invention
The present invention relates to power factor correction in pulse width modulation (PWM) converters. In particular, it relates to a feedback control method for average current mode control to achieve power factor correction in PWM converters without input voltage sensing.
2. Description of the Related Art
Average current mode controlled (ACMC) power factor correction techniques utilizing converters operating in continuous conduction mode (CCM) are the primary choice for many medium and high power applications. A popular control technique utilized to implement ACMC is the three loop architecture as disclosed by P. C. Todd in "UC3854 Controlled Power Factor Correction Circuit Design", Unitrode Product and Applications Handbook, 1995-1996, pp. 10-303.about.322. In accordance with this method, a current programming signal sets the reference for a high bandwidth, fast acting current loop. The amplitude of the current programming signal is slowly modulated by the low bandwidth output voltage error signal and the low pass filtered line voltage rms signal to ensure steady-state input-output power balance. The need for a squarer-divider-multiplier circuit in the control IC to ensure power balance under dynamic changes in load and line rms voltage is explained by L. H. Dixon in "High Power Factor Switching Preregulator Design Optimization", Unitrode Power Supply Design Seminar Manual, SEM 700, 1990. If the current programming signal is not derived from the line explicitly, the advantages in terms of reducing the internal circuitry in the integrated circuit (IC) and external passive components are obvious. Generating the 120 Hz full-wave rectified sine wave current programming signal on-chip is not an attractive solution either.
Methods for achieving high power factor without input voltage sensing for converters operating in CCM have been reported by D. Maksimovic et al., "Nonlinear-carrier Control for High Power Factor Boost Rectifier", APEC 95, pp. 635-641; D. Maksimovic et al., "Nonlinear-carrier Control for High Power Factor Rectifiers based on Flyback, Cuk or Sepic Converters", APEC 96, pp. 814-820; and J. P. Gegner and C. Q. Lee, "Linear Peak Current Mode Control: A Simple Active Power Factor Correction Control Technique for Continuous Conduction Mode", PESC 96, pp. 196-202. Maksimovic et al. utilized the steady-state input-output voltage relationships of boost and flyback topologies along with the average switch current information to derive "non-linear" PWM waveforms that provide high power factor. However, the opportunity of utilizing the average values of other branch currents in the power-stage to derive alternate PWM waveforms was not considered. Moreover, it is exceedingly difficult to design circuits which accurately and consistently produce such "non-linear" PWM waveforms.
Accordingly, there exists a need for an apparatus and method for average current mode control to achieve power factor correction in PWM converters without input voltage sensing, that does not suffer from the foregoing drawbacks.